Lava may have buried signs of Mars water

Dramatic features of the Martian landscape that appear to have experienced catastrophic flooding may have been covered over by lava flows, new research suggests. This could make it much harder for future landing missions to analyse the evidence for past water on the Red Planet.

The finding is one of several new reports resulting from more than three months of high-resolution surveys by the new Mars Reconnaissance Orbiter spacecraft, which went into orbit around the planet in March 2006.

Among the areas examined in detail is a valley system called Athabasca Valles, which has long been interpreted as having been carved out by sudden, catastrophic flooding. The new images show the entire region seems to be covered by a few metres of lava.

Team member Alfred McEwen of the University of Arizona in Tucson, US, still thinks the valley system was created by flowing water. But he says future robotic missions to Mars will probably not be able to investigate the water's effects on the surface there.

Other MRO observations also suggest it will be difficult to interpret the role water has played on the surface of Mars. For example, the planet's extremely flat northern plains, interpreted by many as the remaining basin of an ancient ocean, does not seem to be covered by a deep layer of fine sediments as some had thought.

Instead, it is strewn by large boulders that according to one theoretical model should have been buried under layers of sediment long ago if the ocean had been deep, long-lived and turbulent.

Buried clues

But McEwen says that model might be wrong and that it is still possible that an ocean covered the region. "There are lots of other concepts of Mars oceans and how they formed," he told New Scientist. "Who knows how much evidence of oceans is buried under lava?"

Maria Zuber at MIT in Cambridge, US, says there is still "plenty of evidence" that water played a role in the planet's past and that large quantities of it remain in the poles and frozen in the ground. The new observations just suggest that in some places, "whatever was there has been covered up by volcanism", she told New Scientist.

Lava has previously thwarted scientists trying to study Gusev crater, where the rover Spirit landed in January 2004. Although all the evidence gathered from orbital images suggested Gusev had been an ancient lake, filled by water from channels seen entering the crater, the lander found mostly just beds of lava, which may overlie earlier lake deposits.

Meanwhile, Zuber and her colleagues were able to use the MRO data to determine that the planet's southern polar ice cap contains the largest reservoir of surface water in the inner solar system outside of Earth.

Mars - The Red Planet is full of surprises; learn more in our continually updated special report.

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Thin, concentric lava flow fronts cover a plain near the head of a tributary of Athabasca Valles, which is thought to have been carved by water (Image: NASA/HiRISE/WL Jaeger et al/Science)